Controllable oscillators are a type of component which are very common in modem electronics. Such oscillators can either be voltage controlled, so-called VCOs (Voltage Controlled Oscillator), or current controlled, CCOs (Current Controlled Oscillator). What is controlled in a CCO/VCO is the output frequency of the oscillator.
It is of course important that the frequency of a controllable oscillator can be controlled over a wide range without the amplitude of the signal varying too much, that the noise level of the signal is low, and that the frequency can be high. Taking into account trends in modem electronics, it is also important that a controllable oscillator takes up as little space as possible, and of course that it is as cheap as possible.